An electric appliance such as a digital camera and an ink jet printer includes an operation component which is driven by a motor. An optical sensor such as a photo interrupter is used to detect, for example, an operating speed of the operation component.
A certain optical sensor, among such optical sensors, has a light receiving unit including a phototransistor and obtains a detection signal by converting, to a voltage, a photocurrent flowing through a phototransistor by use of an external resistance. In this manner, a dependency of the detection signal on the photocurrent leads to a dependency of a sensor response on the photocurrent. Thus, an improvement in responsiveness of the sensor is limited. In summary, the optical sensor using a photo interrupter is not suitable for an apparatus, such as a digital camera, for which a high-speed operation is required. Under such circumstance, there is a demand for an optical sensor operable at a high speed without depending on a photocurrent.
For example, Patent Literature 1 discloses an optical sensor (a light-receiving device) configured such that an end of a photodiode and an end of a load are fixed to GND electric potential. Such light-receiving device allows a prevention of a fluctuation in a bias voltage of a photodiode.
Further, Patent Literature 2 discloses an optical sensor configured to bias a photodiode with a gate-source voltage of an nchMOS transistor. Even this type of optical sensor can prevent the fluctuation in the bias voltage of the photodiode.
Both of the optical sensors disclosed in Patent Literatures 1 and 2 can prevent an electric potential difference ΔV between a cathode and an anode of the photodiode by applying the bias voltage to the photodiode. Accordingly, it is possible to decrease an electric charge (ΔQ=C×ΔV) of a capacitance of a photodiode, which capacitance is generated when the photocurrent flowing through the photodiode is converted into a voltage by the resistor. As a result, the optical sensors can operate at a high speed.
This kind of technique can be also applied to the phototransistor so as to allow the high-speed operation of the optical sensor including the phototransistor.